Chapter 9: Gravity (from Lecture Slide) Flashcards
Newton discovered that gravity is
Universal
Newton hypothesized that the moon is
Constantly falling towards Earth
Compared motion of the moon to that of a cannonball being
Fired from the top of a high mountain
If fired fast enough, the cannonball would
Fall around the Earth (satellite)
Aristotle’s time:
Motion of planets and stars is natural
Aristotle’s time not governed by
The same laws as objects on Earth
Newton is recognized that
A force directed toward the Sun because it must act on planets and other celestial bodies
Newtonian synthesis
The same set of laws apply to both celestial and terrestrial objects
Law of Universal Gravitation
Everything pulls on everything else
Every body attracts every other body with a force that is
Directly proportional to the product of their masses and inversely proportional to the square of the distance separating them
The Universal Law of Gravity: Proportionality relationship
1) Constant r
2) Constant m1 and m2
1) Constant r
The greater the masses m1 and m2 of two bodies, the greater the force of attraction between them
2) Constant m1 and m2
The greater the distance of separation r, the weaker the force of attraction
Gravitational force is
The weakest of the 4 fundamental forces
Four fundamental forces are included:
1) Gravitational force
2) Weak force
3) Electromagnetic force
4) Strong force
The gravitational constant G, which links force with mass and distance, is universal because
It is the same everywhere in the universe, and this universality feature pi (r)
The gravitational force between you and the Earth is much
Greater
The weight at the Earth’s surface is
The same as the gravitational force between you and the Earth at Earth’s surface
Acceleration due to gravity, g
1) Directly proportional to the mass of the Earth
2) Inversely proportional to the square of the distance from the center of the Earth
Inverse square law: gravity and distance
1) Intensity of an effect is inversely proportional to the distance from the source
2) Newton law of gravitation, electrostatic, light, sound and radiation phenomena follow an inverse square law
For increase in distance, there are decreases in
Force (intensity)
At great distance, force approaches but never reaches
Zero
The force of gravity between two planets depends on their
Masses and separation distance
Weight
Force an object exert, against a supporting surface
Standing on a scale in an elevator accelerating downward, less compression in scale springs, Weight is:
Less
Standing on a scale in an elevator accelerating upward, more compression in scale springs, Weight is:
Greater
At constant speed in an elevator, no change in
Weight
When you stand still on a bathroom scale that is supported on a stationary floor, this tells that
Your weight is at rest
Gravitational force between you and Earth pulls you
Against the supporting floor and the scale
Newton 3rd laws: the floor and scale push upward on you. Repeat this weighing procedure in a moving elevator, your weight reading varies
Not during steady motion but during accelerated motion
Weightlessness:
No support force as in free fall
Astronauts are without a support force and are continuously feeling
Weightless
Elevator is at rest or moving at constant speed, the weight is
The same
Elevator accelerates upward, floor and scale push
Harder against your feet, so the weight is feel “greater”
Elevator accelerates downward, floor and scale push
Less against your feet weigh feels less
Elevator in free fall and support force and scale reading are
Zero, and you are weightless
There is still a gravitational force (mg) and still accelerates downward (g) but gravity is now
Not feel as weight because there is no support force
The difference between ocean levels at different times of the day are called, what?
Tides
There are typically two high tides and two low tides are what day?
Each day
Ocean tides are caused by
The gravitational attraction of the Moon and the Sun
Unequal tugs on Earth’s ocean causes what?
A stretching effect that produces a pair of ocean bulges
Because the two bulges are on
Opposite sides, and high tides occur approximately every 12 hours
The sun also contributes to ocean tide, but is
Less effective
During the new Moon, or full Moon, the effects of Moon and Sun were
Add up, causing more pronounced spring tides
When the Moon is halfway between a new and full Moon, the tides due to Sun and Moon partly
Cancel each other that causing less pronounced neap tides
Earth and Moon are pull on what?
Each other
How do the Earth and the Moon interact without touching?
The interaction between Earth and Moon is action at a distance
One way to know that Earth is surrounded by
A gravitational field
Moon interacts with the same as Earth, what?
A gravitational field also
Gravitational fields are represented by
Field lines radiating into the object (Earth)
Gravitational field (inward direction)
Indicating that the force is always attractive to Earth
The crowding of arrows closer to Earth indicates that
The magnitude of the force is larger closer to Earth
Gravitational field (symbolized)
Field strength decreases with increasing radii (gr = 1/r^2)
Gravitational field inside the planet, g strength decreases to
Zero at the center
The force due to the upper half of the Earth cancels the force due to
The lower half at the center of the Earth
Similarity, any force due to any position of the Earth at its center will be cancelled by
The portion opposite to it
Outside a planet, it is
Decreases to zero at “infinity” because you are farther away from planet
Suppose you dig a hole through Earth to the other side and jump in, as you fall, your acceleration toward the center will
Go on decreasing
At the center, your acceleration will be zero but your speed is
Maximum
past the center you will be pulled back up, but because
You have acquired sufficient speed you will get to the other side
Einstein’s theory of gravitation: Gravitational speed is
A warping of space-time by any mass and the warped space time affect the motion of other objects
Light emitted by distant object pass by
More massive object in the universe
The gravitational built from these objects can
Distort or bent the light
Strong gravitational lensing results in
Multiple images of these objects
Weak gravitational lensing result in
Objects appearing distorted, stretched, or magnified
When a start exhausts nuclear fuel, it shrinks, so that its mass becomes
Concentrated within a smaller radius
Gravitational force on the surface increases because when
D decreases and F increases
Black hole: When a collapsing star becomes so small that
Its gravitational field exceed the threshold for allowing anything, even light, to escape, a black form is formed, trapping everything inside forever by warped spacetime
Wormhole (definition)
A distortion of space-time
Instead of collapsing toward an infinitely dense point, a wormhole opens out again in
Another part of the universe or in a different universe
Universal gravitation
Everything attracts everything else
Earth is round because of gravitation—all parts of Earth have been
Pulled in, making the surface equidistant from the center
The universe is expanding and accelerating outward, pushed by mysterious
Dark energy
Galaxies exhibit extra gravitational pull attributed to mysterious
Dark matter
